I object to a certain melody (7)
I believe the answer is:
measure
'melody' is the definition.
(thesaurus)
'i object to a certain' is the wordplay.
'i object' becomes 'me'.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'me'+'a'+'sure'='MEASURE'
(Other definitions for measure that I've seen before include "Ascertain the size of" , "Establish the size" , "Ascertain the length etc." , "So much" , "See 1" .)